Chapter 1: An Introduction to Computer Science

1
Chapter 1 An Introduction to Computer Science

• Invitation to Computer Science,
• Java Version, Third Edition

2
Objectives

• In this chapter, you will learn about
• The definition of computer science
• Algorithms

3
Introduction

• Common misconceptions about computer science
• Computer science is the study of computers
• Computer science is the study of how to write
  computer programs
• Computer science is the study of the uses and
  applications of computers and software

4
The Definition of Computer Science

• Gibbs and Tucker definition of computer science
• Computer science is the study of algorithms that
  is used to solve problems.
• The problem solving process involves 4 steps
• Creating correct and efficient algorithms
• Building computers to execute algorithms
• Designing programming languages to translate
  algorithms
• Applications

5
Quiz

• 1. (T/F) Computer science is the study of
  allergy.
• 2. (T/F) Building cars is one of problems solving
  steps used by computer scientists.
• 3. (T/F) Designing programming languages is the
  third step of problems solving steps used by
  computer scientists.

6
The Definition of Computer Science (continued)

• Algorithm
• An ordered step by step instructions that is
  guaranteed to solve a specific problem

7
The Definition of Computer Science (continued)

• An algorithm is a list that looks like
• STEP 1 Do something.
• STEP 2 Do something.
• STEP 3 Do something.
• . .
• . .
• . .
• STEP N Stop. You are finished.

8
Example of Algorithm How to make sticky rice?

• 1. Measure Your Sticky Rice.
• 2. Add Water and Let Soak.
• 3. Add Salt and Bring to a Boil.
• 4. Cook the Rice.
• 5. Use a Fork to Check the Rice.
• 6. Allow Rice to Steam-Cook for 20 min..
• 7. Enjoy Your Sticky Rice!

9
Criteria for Correct Algorithm

• Criteria
• 1. unambiguous instructions
• 2. computable instructions
• 3. producing results
• 4. stopping in a finite amount of time
• Example
• 1. Add X to some numbers -gtviolate criteria
  1
• 2. Set X to (5 / 0) -gtviolate
  criteria 2 and 3
• 3. Repeat Step 4 Until X gt 10 -gtviolate criteria
  4
• 4. Set X 1
• 5. Stop

10
Algorithm Operation 1 Sequential Operation

• Sequential operations
• Carry out a single well-defined task when that
  task is finished, the algorithm moves on to the
  next operation
• Examples
• 1. Set X to 1
• 2. Set Y to 2
• 3. Set Z to X Y
• 4. Print Z
• 5. Stop

11
Quiz

• What is the output of Step 4 of the following
  algorithm?
• 1. Set Y to 4
• 2. Set W to 2
• 3. Set P to (Y W) / 2
• 4. Print P
• 5. Stop

12
Quiz

• What is the output of Step 4 of the following
  algorithm?
• 1. Set Y to 4
• 2. Set W to Y 2
• 3. Set Y to (Y W) / 2
• 4. Print Y
• 5. Stop

13
Algorithm Operation 1 Conditional Operation

• Conditional operations
• Ask a question and then select the next operation
  to be executed on the basis of the answer to that
  question
• Examples
• 1. Set X to 1
• 2. If X is even
• then Set Y to 1
• else Set Y to 2
• 3. Stop

14
Quiz

• What is the output of Step 3 of the following
  algorithm?
• 1. Set X to 4
• 2. If X gt 3
• then Set Y to A
• else Set Y to B
• 3. Print Y
• 4. Stop

15
The Definition of Computer Science (continued)

• Iterative operations
• Go back and repeat the execution of a previous
  block of instructions
• Examples
• 1. Set X to 1
• 2. Repeat Step3 Until X is 3
• 3. Set X to X 1
• 4. Stop

16
Quiz

• What is the output of Step 3 of the following
  algorithm?
• 1. Set X to 1
• 2. Set Y to 2
• 3. Repeat Step4-5 Until X is 3
• 4. Set X to X 1
• 5. Set Y to Y 2
• 6. Stop

17

• What is the output of Step 6 of the following
  algorithm?_____
• How many times have step 4 been executed? ____
• 1. Set X to 1
• 2. Set Y to 2
• 3. Repeat Step4-5 Until X is Y gt 7
• 4. Set X to X 3
• 5. Set Y to Y 3
• 6. Print Y
• 7. Stop

18
Quiz

• What is the output of Step 6 of the following
  algorithm?_____
• How many times have step 4 been executed? ____
• 1. Set X to 1
• 2. Set Y to 2
• 3. Repeat Step4-5 Until X is Y gt 7
• 4. Set X to X 3
• 5. Set Y to Y 3
• 6. Print X
• 7. Stop

19
What is boolean?

• a boolean is either TRUE or FALSE
• What is the output of the following algorithm?___
• 1. Set B to FALSE
• 2. If B is TRUE then
• Set A to OK
• else
• Set A to YES
• 3. Print A
• 4. Stop

20
Quiz

• What is the output of step 4 of the following
  algorithm?___
• Has the statement Set A to TRUE been executed?
  _____
• 1. Set B to TRUE
• 2. If B is TRUE then
• Set A to FALSE
• Else
• Set A to TRUE
• 3. If A is FALSE then
• Set W to X
• Else
• Set W to Y
• 4. Print W
• 5. Stop

21
What is the output on step 6? ________ How many
times of step 4 been executed? _______ 1. Set
INDEX to 0 2. Set DONE to false 3. Repeat steps
4-5 until DONE is true 4. Set INDEX to INDEX
1 5. If INDEX gt 2 then Set DONE to
true 6. Print INDEX 7. Stop
22
Quiz

• What is the output on step 6? ________
• How many times of step 4 been executed? _______
• 1. Set INDEX to 1
• 2. Set DONE to false
• 3. Repeat steps 4-5 until DONE is true
• 4. Set INDEX to INDEX 2
• 5. If INDEX gt 5 then
• Set DONE to true
• 6. Print INDEX
• 7. Stop

23
Repeat/Until

• Repeat A Until B
• meaning if B is not true then do A.

24
While /Do

• While A Do B
• meaning if A is true then do B

25

• What is the output of step 6?_____
• How many times have step5 been executed? ____
• 1. Set INDEX to 1
• 2. Set SUM to 1
• 3. While INDEX lt 5 do steps 4 - 5
• 4. Set SUM to SUM INDEX
• 5. Set INDEX to INDEX 2
• 6. Print SUM
• 7. Stop

26
Quiz

• What is the output of step 6?_____
• How many times have step5 been executed? ____
• 1. Set INDEX to 2
• 2. Set SUM to 3
• 3. While INDEX lt 8 do steps 4 - 5
• 4. Set SUM to SUM INDEX
• 5. Set INDEX to INDEX 3
• 6. Print SUM
• 7. Stop

27

• What is the output of step 8?_____
• How many times have step5 been executed? ____
• 1. Set INDEX to 1
• 2. Set SUM to 1
• 3. Set DONE to false
• 4. While DONE is false do steps 5-7
• 5. Set SUM to SUM INDEX
• 6. Set INDEX to INDEX 3
• 7. If INDEX gt 7 THEN
• Set DONE to true
• 8. Print SUM
• 9. Stop

28
Quiz

• What is the output of step 8?_____
• How many times have step5 been executed? ____
• 1. Set INDEX to 1
• 2. Set SUM to 1
• 3. Set DONE to false
• 4. While DONE is false do steps 5-7
• 5. Set SUM to SUM INDEX
• 6. Set INDEX to INDEX 3
• 7. If INDEX gt 7 THEN
• Set DONE to true
• 8. Print SUM
• 9. Stop

29
Quiz - Challenge

• What is the output on step 6? ________
• How many times of step 4 been executed? _______
• 1. Set INDEX to 0
• 2. Set DONE to true
• 3. Repeat steps 4-5 until DONE is true
• 4. Set INDEX to INDEX 1
• 5. If INDEX gt 2 then
• Set DONE to true
• 6. Print INDEX
• 7. Stop

30
Quiz

• What is the output of step 8?_____
• How many times have step5 been executed? ____
• 1. Set INDEX to 1
• 2. Set SUM to 1
• 3. Set DONE to true
• 4. While DONE is false do steps 5-7
• 5. Set SUM to SUM INDEX
• 6. Set INDEX to INDEX 3
• 7. If INDEX gt 7 THEN
• Set DONE to true
• 8. Print SUM
• 9. Stop

31
Chapter 1. Exercises Samples

• Exercise 5.
• Under what conditions would the following
  function not be effectively computable?
• F(x) sqrt(a)/b

32
Chapter 1. Exercises Samples

• Exercise 7. What is the output of the following
  algorithm if X is 5 and Y is 2?
• 1. Get X
• 2. Get Y
• 3. Divide X by Y and call the reminder R
• 4. If R is not 0 then reset X to X-1 and go back
  to step 3.
• 5. Print X
• 6. Stop

33
Exercise 9 Samples

• Write an algorithm to compute the area of a
  rectangle.
• Area Length Width
• Get Length
• Get Width
• Set Area Length Width
• Print Area
• Stop

34
Quiz

• Write an algorithm to compute the area of a
  triangle.
• Rule Area Height Width / 2

35
Exercise 9 Sample.

• Write an algorithm to determine the financial
  status.
• Rule If the savings gt 10000 then output Good
  otherwise Bad.
• Get Savings
• If Savings gt 10000
• Set Status Good
• Else
• Set Status Bad
• Print Status
• Stop

36
Quiz

• Write an algorithm to determine the weather
  status.
• Rule If the temperature lt 32F then output Cold
  otherwise Warm.

37
Quiz

• Write an algorithm to compute the following
• mysterious equation. Be sure to print the error
  message when C is 0.
• rule A B / C

38
Example. If/else

• Write an algorithm to compute and display the
  value sqrt(x) if the value of x is not negative.
  If x is negative, then display the message
  Unable to perform the sqrt() operation.
• 1. Get X
• 2. If X lt 0
• Print Unable to perform the sqrt()
• Else
• Print sqrt(x)
• 3. Stop

39
Quiz

• Write an algorithm to compute and display the
  value a/b if the value of b is not 0. If b is 0,
  then display the message Unable to perform the
  division.

40
Loop Exercise 1. The worst solution.

• Write an algorithm to compute
• Sum 234?
• 1. Set Sum to 0
• 2. Set Num to 2
• 3. Set Sum Sum Num
• 4. Set Num to Num 1
• 5. Set Sum Sum Num
• 6. Set Num to Num 1
• 7. Set Sum Sum Num
• 8. Print Sum
• 9. Stop

41
Loop Exercise 2. The best solution

• Write an algorithm to compute
• Sum 234?

• 1. Set Sum to 0
• 2. Set Count to 1
• 3. Set Num to 2
• 4. While Count lt3 Do Steps 5-7
• 5. Sum Sum Num
• 6. Num Num 1
• 7. Count Count 1
• 8. Print Sum
• 9. Stop

42
Loop Exercise 2. The best solution

• Write an algorithm to compute
• Sum 234?

1. Set Sum to 0 2. Set Count to 1 3. Set Num to
2 4. Repeat Sept 5-7 Until Count gt 3 Do 5.
Sum Sum Num 6. Num Num 1 7.
Count Count 1 8. Print Sum 9. Stop
43
Quiz. Loop.

• Use While/Do to write an algorithm to compute
• sum 1 2 3 4 5 6 7 8 9 10

44
Quiz. Loop.

• Use Repeat/Until to write an algorithm to compute
  
• sum 1 2 3 4 5 6 7 8 9 10

45

• Use While/Do to write an algorithm to compute
• sum 2 4 6 8 10

46
The Definition of Computer Science (continued)

• Iterative operations
• Tell us to go back and repeat the execution of a
  previous block of instructions
• Examples
• Repeat the previous two operations until the
  mixture has thickened
• While there are still more checks to be
  processed, do the following five steps
• Repeat steps 1, 2, and 3 until the value of y is
  equal to 11

47
The Definition of Computer Science (continued)

• If we can specify an algorithm to solve a
  problem, we can automate its solution
• Computing agent
• The machine, robot, person, or thing carrying out
  the steps of the algorithm
• Does not need to understand the concepts or ideas
  underlying the solution

48
The Formal Definition of an Algorithm

• Algorithm
• A well-ordered collection of unambiguous and
  effectively computable operations that, when
  executed, produces a result and halts in a finite
  amount of time
• Unambiguous operation
• An operation that can be understood and carried
  out directly by the computing agent without
  needing to be further simplified or explained
• Ambiguous example, Set X to some number

49
The Formal Definition of an Algorithm (continued)

• A primitive operation (or a primitive) of the
  computing agent
• Operation that is unambiguous for computing agent
• Primitive operations of different individuals (or
  machines) vary
• An algorithm must be composed entirely of
  primitives
• Effectively computable
• Computational process exists that allows
  computing agent to complete that operation
  successfully
• Ineffectively computable example, Generate a
  list of prime numbers

50
The Formal Definition of an Algorithm (continued)

• The result of the algorithm must be produced
  after the execution of a finite number of
  operations
• Infinite loop
• The algorithm has no provisions to terminate
• A common error in the designing of algorithms
• Infinite loop example,
• Step 1. Wet the car
• Step 2. Repeat Step 3
• Step 3. Wash the car

51
The Importance of Algorithmic Problem Solving

• Algorithmic solutions can be
• Encoded into some appropriate language
• Given to a computing agent to execute
• The computing agent
• Would mechanically follow these instructions and
  successfully complete the task specified
• Would not have to understand
• Creative processes that went into discovery of
  solution
• Principles and concepts that underlie the problem

52
The Early Period Up to 1940

• 3,000 years ago Mathematics, logic, and
  numerical computation
• Important contributions made by the Greeks,
  Egyptians, Babylonians, Indians, Chinese, and
  Persians
• 1614 Logarithms
• Invented by John Napier to simplify difficult
  mathematical computations
• Around 1622 First slide rule created

53
The Early Period Up to 1940 (continued)

• 1672 The Pascaline
• Designed and built by Blaise Pascal
• One of the first mechanical calculators
• Could do addition and subtraction
• 1674 Leibnitzs Wheel
• Constructed by Gottfried Leibnitz
• Mechanical calculator
• Could do addition, subtraction, multiplication,
  and division

54

• Figure 1.4
• The Pascaline One of the Earliest Mechanical
  Calculators

55
The Early Period Up to 1940 (continued)

• 1801 The Jacquard loom
• Developed by Joseph Jacquard
• Automated loom
• Used punched cards to create desired pattern
• 1823 The Difference Engine
• Developed by Charles Babbage
• Did addition, subtraction, multiplication, and
  division to 6 significant digits
• Solved polynomial equations and other complex
  mathematical problems

56
The Early Period Up to 1940 (continued)

• 1823 The Difference Engine
• Developed by Charles Babbage
• Capabilities
• Addition, subtraction, multiplication, and
  division to 6 significant digits
• Solve polynomial equations and other complex
  mathematical problems

57

• Figure 1.5
• Drawing of the Jacquard Loom

58
The Early Period Up to 1940 (continued)

• 1830s The Analytic Engine
• Designed by Charles Babbage
• More powerful and general-purpose computational
  machine
• Components were functionally similar to the four
  major components of todays computers
• Mill (modern terminology arithmetic/logic unit)
• Store (modern terminology memory)
• Operator (modern terminology processor)
• Output (modern terminology input/output)

59
The Early Period Up to 1940 (continued)

• 1890 U.S. census carried out with programmable
  card processing machines
• Built by Herman Hollerith
• These machines could automatically read, tally,
  and sort data entered on punched cards

60
The Birth of Computers 19401950

• Development of electronic, general-purpose
  computers
• Did not begin until after 1940
• Was fueled in large part by needs of World War II
• Early computers
• Mark I
• ENIAC
• ABC system
• Colossus
• Z1

61

• Figure 1.6
• Photograph of the ENIAC Computer

62
The Birth of Computers 19401950

• Stored program computer model
• Proposed by John Von Neumann in 1946
• Stored binary algorithm in the computers memory
  along with the data
• Is known as the Von Neumann architecture
• Modern computers remain, fundamentally, Von
  Neumann machines
• First stored program computers
• EDVAC
• EDSAC

63
The Modern Era 1950 to the Present

• First generation of computing (1950-1959)
• Used vacuum tubes to store data and programs
• Each computer was multiple rooms in size
• Computers were not very reliable

64
The Modern Era 1950 to the Present (continued)

• Second generation of computing (1959-1965)
• Replaced vacuum tubes by transistors and magnetic
  cores
• Dramatic reduction in size
• Computer could fit into a single room
• Increase in reliability of computers
• Reduced costs of computers
• High-level programming languages
• The programmer occupation was born

65
The Modern Era 1950 to the Present (continued)

• Third generation of computing (1965-1975)
• Used integrated circuits rather than individual
  electronic components
• Further reduction in size and cost of computers
• Computers became desk-sized
• First minicomputer developed
• Software industry formed

66
The Modern Era 1950 to the Present (continued)

• Fourth generation of computing (1975-1985)
• Reduced to the size of a typewriter
• First microcomputer developed
• Desktop and personal computers common
• Appearance of
• Computer networks
• Electronic mail
• User-friendly systems (Graphical user interfaces)
• Embedded systems

67

• Figure 1.7
• The Altair 8800, the Worlds First Microcomputer

68
The Modern Era 1950 to the Present (continued)

• Fifth generation of computing (1985-?)
• Recent developments
• Massively parallel processors
• Handheld devices and other types of personal
  digital assistants (PDAs)
• High-resolution graphics
• Powerful multimedia user interfaces incorporating
  sound, voice recognition, touch, photography,
  video, and television

69
The Modern Era 1950 to the Present (continued)

• Recent developments (continued)
• Integrated global telecommunications
  incorporating data, television, telephone, FAX,
  the Internet, and the World Wide Web
• Wireless data communications
• Massive storage devices
• Ubiquitous computing

70

• Figure 1.8
• Some of the Major Advancements in Computing

71

• Figure 1.8
• Some of the Major Advancements in Computing

72
Organization of the Text

• This book is divided into six separate sections
  called levels
• Each level addresses one aspect of the definition
  of computer science
• Computer science/Algorithms

73
Organization of the Text

• Level 1 The Algorithmic Foundations of Computer
  Science
• Chapters 1, 2, 3
• Level 2 The Hardware World
• Chapters 4, 5
• Level 3 The Virtual Machine
• Chapters 6, 7

74
Organization of the Text

• Level 4 The Software World
• Chapters 8, 9, 10, 11
• Level 5 Applications
• Chapters 12, 13, 14
• Level 6 Social Issues
• Chapter 15

75

• Figure 1.9
• Organization of the Text into a Six-Layer
  Hierarchy

76
Summary

• Computer science is the study of algorithms
• An algorithm is a well-ordered collection of
  unambiguous and effectively computable operations
  that, when executed, produces a result and halts
  in a finite amount of time
• If we can specify an algorithm to solve a
  problem, then we can automate its solution
• Computers developed from mechanical calculating
  devices to modern electronic marvels of
  miniaturization